The design of a charger for any cell or group of cells is always a compromise between the requirements for the intended use and the characteristics of the battery. Even the definition of rechargeability depends on the specific application. For example, a device which discharges a moderate capacity battery at a moderate rate, such as a portable radio or tape player, could be powered by ordinary alkaline zinc manganese dioxide cells. While these cells are generally not considered rechargeable, it is possible to charge them with a special purpose, sophisticated battery charger.
NiCd cells are preferred in applications requiring relatively high discharge rates after prolonged charging at elevated temperature, as in fluorescent emergency lights. Chargers are presently available to charge NiCd batteries. However, NiCd batteries are toxic and there use is being restricted by law. Moreover, battery chargers which are capable of charging NiCd batteries cannot charge NiMH batteries.
NiMH cells are often selected where capacity per unit weight or volume, rather than peak power or disposal safety, are critical. These cells can be charged by presently available battery chargers. However, battery chargers which are capable of charging NiMH cells are generally not appropriate for charging NiCd cells and vice versa. As NiCd batteries are phased out, electronic devices such as portable computers can use two different types of batteries, i.e. NiCd and NiMH. However, the same battery charger typically can not charge both. Further, if one type of battery were placed in a charger suitable for the other type of battery, the battery and possibly the charger itself would be damaged. Since these batteries are relatively expensive, it is desirable to prevent such an occurrence. Moreover, it would be highly desirable to provide a single battery charger which could effectively charge both NiCd and NiMH batteries interchangeably.